Methods and apparatus for analyzing effects of friction on process control devices

ABSTRACT

Methods and apparatus for analyzing effects of friction on process control devices are disclosed herein. An example apparatus includes a housing; a processor disposed in the housing, the processor to: determine a first force or torque corresponding to friction of a process control device and an actuator operatively coupled to the process control device via a stem or shaft; in response to the first force or torque, determine a first command to operate the process control device via the stem or shaft to obtain a first response of the actuator; determine a second force or torque corresponding to friction of the process control device and the actuator; and in response to the second force or torque, determine a second command to operate the process control device via the stem or shaft to obtain a second response of the actuator.

RELATED APPLICATION

This patent arises from a continuation of U.S. patent application Ser.No. 13/451,862, filed on Apr. 20, 2012, which is hereby incorporatedherein by reference in its entirety.

FIELD OF THE DISCLOSURE

This disclosure relates generally to process control devices and, moreparticularly, to methods and apparatus for analyzing effects of frictionon process control devices.

BACKGROUND

Process control systems generally use a variety of process controldevices (e.g., rotary valves, linear valves, etc.) to control a process.The process control devices are often operated by an actuator via a stemor shaft. During operation, friction of the process control device andthe actuator resists movement of the stem or shaft. Over time, thefriction of the process control device and the actuator may increase ordecrease.

SUMMARY

An example apparatus includes a housing; a processor disposed in thehousing, the processor to: determine a first force or torquecorresponding to friction of a process control device and an actuatoroperatively coupled to the process control device via a stem or shaft;in response to the first force or torque, determine a first command tooperate the process control device via the stem or shaft to obtain afirst response of the actuator; determine a second force or torquecorresponding to friction of the process control device and theactuator; and in response to the second force or torque, determine asecond command to operate the process control device via the stem orshaft to obtain a second response of the actuator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example process control system within which theteachings of this disclosure may be implemented.

FIG. 2 depicts an example process control device that may be used toimplement example methods disclosed herein.

FIG. 3 is a flow chart representative of an example method disclosedherein.

FIG. 4 is a flow chart representative of another example methoddisclosed herein.

DETAILED DESCRIPTION

While the following example apparatus and methods are described inconjunction with a linear valve and a linear actuator, the exampleapparatus and methods may also be used with any other process controldevice operated by a linear or rotary actuator such as, for example,throttle valves, isolation valves, rotary valves, and/or any otherprocess control device.

Industrial processes (e.g., oil and gas distribution systems, chemicalprocessing plants, etc.) are often controlled by a variety of processcontrol devices (e.g., linear valves, rotary valves, throttle valves,isolation valves, etc.). A process control device is often operated byan actuator via a stem or shaft. The actuator provides a force or torqueto the stem or shaft to move a flow control member (e.g., a plug, adisk, a ball, etc.) of the process control device. During operation,fluid flows through the process control device and subjects the flowcontrol member and, thus, the stem or shaft to a variety of forces.Also, during operation, friction of the process control device and theactuator resists movement of the stem or shaft. If the frictionincreases, an amount of movement of the stem or shaft in response to agiven amount of force of torque of the actuator may vary betweenoperational cycles, the movement may be sluggish, or the stem or shaftmay not move in response to the force or torque provided by theactuator. If the friction decreases, the amount of movement of the stemor shaft in response to a given amount force or torque of the actuatormay increase.

The examples disclosed herein may be used to analyze or assess an effectof friction on operation of a process control device operated by anactuator. In some examples, the effect of the friction on the operationof the process control device is exhibited as a deterioration of amovement characteristic of a stem or shaft of the actuator, which may beindicative of an impairment or disability of the process control deviceor the actuator (e.g., wear of a seal, inadequate supply air pressure tothe actuator, blockage of a flow path of the process control device,etc).

An example method disclosed herein includes determining a valuecorresponding to friction of the actuator and the process control deviceoperated by the actuator. In some examples, the value corresponding tothe friction is a difference between a force or torque applied by theactuator to the stem or shaft at a first position and a force or torqueapplied by the actuator to the stem or shaft at a second position duringmovement of the stem from the first position to the second position(i.e., a differential force corresponding to dynamic friction opposingthe movement of the stem from the first position to the secondposition). The example method further includes determining a valueindicative of an effect of the friction on operation of the processcontrol device via the actuator based on the value corresponding to thefriction and a predetermined value.

The effect of the friction on the operation of the process controldevice may correspond to a movement characteristic of the stem or shaftof the process control device such as, for example, a sensitivity, apreciseness, and/or a responsiveness of movement of the stem or shaft inresponse to a force or torque applied by the actuator to the stem orshaft. In some examples, the predetermined value is a difference betweena maximum force or torque of the actuator and a force or torque of theactuator to perform an action. To determine the value indicative of theeffect of the friction, the value corresponding to the friction may becompared to the predetermined value using, for example, a ratio. Analert message may be sent when the ratio indicates that the effect ofthe friction has reached a predetermined level.

FIG. 1 illustrates an example process control system 100 that may beused to implement the example apparatus and methods disclosed herein.The example process control system 100 includes any number of processcontrol devices 102 such as input devices and/or output devices. In someexamples, the input devices include valves, pumps, fans, heaters,coolers, mixers, and/or other devices, and the output devices includethermometers, pressure gauges, concentration gauges, fluid level meters,flow meters, vapor sensors, valve controllers, and/or other devices. Theinput and output devices are communicatively coupled to a controller 104(e.g., a DeltaV™ controller) via a data bus (e.g., Foundation Fieldbus™and HART™) or local area network (LAN) 106. The input and output devicesmay be wirelessly communicatively coupled to the controller 104. Thecontroller 104 transmits instructions to the input devices to controlthe process and receives and/or collects information (e.g., measuredprocess information, environmental information, and/or input deviceinformation, etc.) transmitted by the output devices. The controller 104generates notifications, alert messages, and/or other information. Thecontroller 104 is also communicatively coupled to a workstation 108,which includes an interface 110 that displays process controlinformation (e.g., measured process control information, alert message,etc.). Although a single controller 104 is shown in FIG. 1, one or moreadditional controllers may be included in the example system 100 withoutdeparting from the teachings of this disclosure.

FIG. 2 depicts an example process control device 200 that may be used toimplement the examples disclosed herein. The example process controldevice 200 depicted in FIG. 2 is a linear valve (e.g., a Fisher® EDValve). However, other process control devices such as, for example,rotary valves (e.g., a Fisher® Vee-Ball™ V150 valve, a Fisher® Vee-Ball™V300 valve, etc.), throttle valves, isolation valves, and/or otherprocess control devices may be used to implement the examples disclosedherein. The example process control device 200 includes a flow controlmember (not shown) (e.g., a plug, a disk, a ball, etc.) coupled to astem 202. A linear actuator 204 (e.g., Fisher® 667 Actuator) isoperatively coupled to the stem 202 to move the stem 202 duringoperation. Some example process control devices that may be used toimplement the examples disclosed herein include a rotary valve or otherdevice operatively coupled to a rotary actuator. In the illustratedexample, the actuator 204 is a pneumatic actuator. However, otheractuators such as, for example, electric or hydraulic actuators may beused to implement the examples disclosed herein. An estimated maximumforce or torque of the actuator 204 may be determined based oncharacteristics of the actuator 204 and the process control system 100such as, for example, an air supply pressure available, an effectivearea of the actuator 204, a lever arm length and/or othercharacteristics. The example process control device 200 also includes aseal (not shown).

In the illustrated example, the process control device 200 includes adigital valve controller 206 (“DVC”) (e.g., Fisher® FIELDVUE™ DVC6200Digital Valve Controller) to collect information such as, for example, aposition of the stem 202, a direction of stem travel, a count ofoperational cycles, actuator pressures, a force or torque provided bythe actuator 204 and/or other information. The DVC 206 iscommunicatively coupled to the actuator 204 and the controller 104.During operation, the DVC 206 transmits the information to thecontroller 104 and receives information from the controller 104. Basedon the information, the DVC 206 transmits commands to the actuator 204(e.g., via a pneumatic signal).

In response to a command from the DVC 206, the actuator 204 applies aforce to the stem 202 to perform an action (e.g., move the flow controlmember, sealingly engage or disengage the flow control member and aseat, etc.). Friction of the process control device 200 and the actuator204 resists movement of the stem 202. Factors affecting the amount offriction include, for example, valve design, actuator effective area,valve port size, trim unbalance area, stem size, shutoff classificationof the valve, valve seat type, and/or other factors. Process conditions(e.g., valve inlet pressure, valve outlet pressure, direction of fluidflow, etc.) may also resist or encourage the movement of the stem 202.

Thus, when the process control device 200 is online (i.e., being used tocontrol the process), the force applied to the stem 202 to perform theaction is a sum or net force needed to overcome the friction of theprocess control device 200 and the actuator 204; a force on the stem 202caused by the process conditions; and any additional forces to performthe action (e.g., a force to compress an actuator spring, a force toachieve a desired shutoff classification, etc.). An estimated force ofthe actuator 204 needed to perform the action may be determined based onestimated process conditions (e.g., estimated valve inlet pressure,estimated valve outlet pressure, etc.) and/or characteristics of theaction such as, for example, shutoff classification desired, stem traveldistance, etc.

Friction affects operation of the process control device 200 via theactuator 204. More specifically, over time the friction of the processcontrol device 200 and/or the actuator 204 may increase or decrease dueto, for example, wear, flow path blockages, process media accumulation,etc. As a result, movement characteristics of the stem 202 in responseto forces applied to the stem 202 by the actuator 204 may change ordeteriorate, thereby affecting a response of the actuator 204 and, thus,the process control device 200 to a command to operate the processcontrol device 200 via the stem 202. Further, a range of suitablecontroller tuning speeds may decrease.

In some examples, one of the movement characteristics affected by thefriction of the process control device 200 and the actuator 204 is asensitivity of movement of the stem 202. In such examples, if thefriction increases, a force to move the stem 202 a first distance mayincrease during subsequent operational cycles. For example, at a firstamount of friction, the stem 202 may move the first distance only whenthe actuator 204 provides at least a first amount of force. After aplurality of operational cycles (e.g., 10,000), the friction of theprocess control device 200 and the actuator 204 may increase to a secondamount of friction, and the stem 202 may move the first distance onlywhen the actuator 204 provides at least a second amount of force that isgreater than the first amount of force to the stem 202. In someexamples, a minimum increment of travel of the stem 202 in response tothe given force provided by the actuator 204 increases for subsequentoperational cycles as the friction increases. For example, the stem 202may move in increments only as small as, for example, a distancecorresponding to 0.25 percent of total possible travel or span at thefirst amount of friction. However, at the second amount of friction, thestem 202 may move in increments only as small as, for example, adistance corresponding to two percent of total possible travel.

In some examples, one of the movement characteristics affected by thefriction of the process control device 200 and the actuator 204 is apreciseness of the movement of the stem 202. In such examples, an amountof movement traveled by the stem 202 in response to the force providedby the actuator 204 may vary or become more variable as the frictionincreases. For example, at the first amount of friction, the stem 202may move a distance of about 0.50 inches in response to the first amountof force applied to the stem 202 by the actuator 204. However, if thefriction increases to the second amount of friction that is greater thanthe first amount of friction, the amount of movement of the stem 202 mayvary between, for example, about 0.35 inches and 0.50 inches in responsethe first amount of force for subsequent operational cycles.

In some examples, one of the movement characteristics affected by thefriction of the process control device 200 and the actuator 204 is aresponsiveness of movement of the stem 202. In such examples, as thefriction increases, a time to initially move the stem 202 in response tothe force provided to the stem 202 by the actuator 204 increases duringsubsequent operational cycles. For example, at the first amount offriction, the stem 202 may initially move a first amount of time afterthe first amount of force is applied to the stem 202. At the secondamount of friction, the stem 202 may initially move a second amount oftime that is greater than the first amount of time after the firstamount of force is applied to the stem 202. In some examples, when thefriction increases from the first amount of friction to the secondamount of friction, a time to move the stem 202 over a distance (e.g.,an inch) increases (i.e., the stem 202 moves slower) when, for example,the first amount of force is applied to the stem 202.

FIGS. 3-4 are flowcharts representative of example methods disclosedherein. Some or all of the example methods of FIGS. 3-4 may be carriedout by a processor, the controller 104 and/or any other suitableprocessing device. In some examples, some or all of the example methodsof FIGS. 3-4 are embodied in coded instructions stored on a tangiblemachine accessible or readable medium such as a flash memory, a ROMand/or random-access memory RAM associated with a processor.Alternatively, some or all of the example methods of FIGS. 3-4 may beimplemented using any combination(s) of application specific integratedcircuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)), fieldprogrammable logic device(s) (FPLD(s)), discrete logic, hardware,firmware, etc. Also, one or more of the operations depicted in FIGS. 3-4may be implemented manually or as any combination of any of theforegoing techniques, for example, any combination of firmware,software, discrete logic and/or hardware. Further, although the examplemethods are described in reference to the flowcharts illustrated inFIGS. 3-4, many other methods of implementing the example methods may beemployed. For example, the order of execution of the blocks may bechanged, and/or some of the blocks described may be changed, eliminated,sub-divided, or combined. Additionally any or all of the example methodsof FIGS. 3-4 may be carried out sequentially and/or carried out inparallel by, for example, separate processing threads, processors,devices, discrete logic, circuits, etc.

While the following example methods of FIGS. 3-4 are described inconjunction with a linear valve and a linear actuator 204, other examplemethods may be implemented using any other process control deviceoperated by a linear actuator or a rotary actuator such as, for example,throttle valves, isolation valves, rotary valves, and/or any otherprocess control device.

With reference to FIGS. 1 and 2, the example method or process 300 ofFIG. 3 begins by determining a value corresponding to friction of theactuator 204 and the process control device 200 operated by the actuator204 (block 302). In some examples, determining the value correspondingto the friction includes determining a difference between a forceapplied to the stem 202 by the actuator 204 at a first position and aforce applied to the stem 202 by the actuator 204 at a second positionduring movement of the stem 202 from the first position to the secondposition. The first and second forces provided by the actuator 204 maybe determined by the DVC 206 when the process control device 200 isonline or offline.

At block 304, a value indicative of an effect of the friction onoperation of the process control device 200 via the actuator 204 isdetermined based on the value corresponding to the friction and apredetermined value. In some examples, the effect of the friction on theoperation of the process control device 200 is a change or deteriorationof a movement characteristic of the stem 202 in response to a forceapplied by the actuator 204 to the stem 202. For example, to operate theprocess control device 200, the DVC 206 sends a command to the actuator204 (e.g., via a pneumatic signal), which provides the force to the stem202 based on the command. The movement characteristic may be asensitivity, a preciseness, or a responsiveness of movement of the stem202 in response to the force provided by the actuator 204. If thefriction increases or decreases during operation, the effect of thefriction on the operation may be an increase or a decrease of thesensitivity, the preciseness, and/or the responsiveness of the movementof the stem 202 in response to the force provided by the actuator 204.

In the illustrated example, the predetermined value is an estimatedforce. In some examples implemented using a rotary actuator, thepredetermined value is an estimated torque. The estimated force ortorque may be a difference between an estimated maximum force or torqueof the actuator 204 and an estimated force or torque of the actuator 204to perform an action (e.g., move the flow control member, sealinglyengage or disengage the flow control member and a seal, etc.). Theestimated maximum force or torque of the actuator 204 is determinedbased on characteristics of the actuator 204, the stem 202 and/or theprocess control system 100 such as, for example, a supply pressureavailable, an actuator effective area, a lever arm length, a maximumshear strength of the stem 202 and/or other characteristics. Theestimated force or torque of the actuator 204 to perform the action maybe determined based on estimated process conditions (e.g., estimatedvalve inlet pressure, estimated valve outlet pressure) and/orcharacteristics of the action such as, for example, shutoffclassification desired, stem travel distance, etc.

The value indicative of the effect of the friction on the operation ofthe process control device 200 may be determined by comparing the valuecorresponding to the friction to the estimated force via, for example, aratio. The ratio of the estimated force over the value corresponding tothe friction may equal, for example, 15, which may indicate that thestem 202 moves a distance corresponding to 0.25 percent of totalpossible stem travel in response to the first amount of force applied tothe stem 202 via the actuator 204. However, if the ratio subsequentlyequals 5, the ratio may indicate that the movement characteristic hasdeteriorated. For example, the ratio equaling 5 may indicate that thestem 202 moves the distance corresponding to 0.25 percent of totalpossible stem travel in response to the second amount of force that isgreater than the first amount of force.

At block 306, it is determined whether the value indicative of theeffect of the friction indicates that the effect of the friction hasreached a predetermined level. For example, the predetermined level maybe indicated by the ratio being less than or equal to 15. If the valueindicates that the effect of the friction has reached the predeterminedlevel, then an alert message is sent (block 308). For example, the DVC206 and/or the controller 104 generates and sends the alert message tothe workstation 108 if the ratio equals 15 or less.

If the value indicates the effect of the friction has not reached thepredetermined level, then a rate of change of the value indicative ofthe effect of the friction is determined (block 310). In some examples,determining the rate of change of the value indicative of the effect ofthe friction is based on the value indicative of the effect of thefriction determined at block 304 and one or more values indicative ofthe effect of the friction. For example, during operation, the processcontrol device 200 may undergo a plurality of operational cycles inwhich the actuator 204 operates the process control device 200 via thestem 202 (e.g., outputs a force to move the stem 202). The DVC 206and/or the controller 104 may determine and/or log in a table ordatabase a frequency of the force outputs and a value indicative of theeffect of the friction corresponding to the force output for each of theoperational cycles. If, at block 306, the value indicates the effect ofthe friction determined at block 304 has not reached the predeterminedlevel then, at block 310, the frequency, the value determined at block304, and the values from the table or database may be used to determinethe rate of change of the value indicative of the effect of thefriction. For example, corresponding to force outputs over about a fivehour period of time, the values from the table or database may be 18.5,18.4, 18.3, and 18.15 and the value determined at block 304 may be 18.0.As a result, the DVC 206 or the controller 104 determines that thefrequency of the force outputs is 1 per hour and the rate of change ofthe value is, thus, about 0.1 per hour.

Based on the rate of change and the value indicative of the effect ofthe friction, a time remaining until the value indicates the effect ofthe friction has reached the predetermined level is determined (block312). For example, if the value is 18, the rate of change of the valueis 0.1 per hour and the predetermined level is indicated by the valueequaling 15, then the time remaining until the value indicates theeffect of the friction has reached the predetermined level is 30 hours.

At block 314, whether the time is less than or equal to a predeterminedtime is determined. If the time is greater than the predetermined time,the example method 300 returns to block 302. If the time is equal to orless than the predetermined time, an alert message is sent (block 308).For example, the DVC 206 and/or the controller 104 generates and sendsan alert message to the workstation 108 if the time is equal to or lessthan one day and/or any other suitable time.

FIG. 4 is a flowchart representative of another example method disclosedherein. With reference to FIGS. 1 and 2, the example method or process400 of FIG. 4 begins by determining a value corresponding to friction ofthe process control device 200 and the actuator 204, which isoperatively coupled to the process control device 200 via the stem 202(block 402). In some examples, determining the value corresponding tothe friction includes determining a difference between a force appliedto the stem 202 by the actuator 204 at a first position and a forceapplied to the stem 202 by the actuator 204 at a second position duringmovement of the stem 202 from the first position to the second position.The first and second forces provided by the actuator 204 may bedetermined by the DVC 206 when the process control device 200 is onlineor offline.

At block 404, based on the force corresponding to the friction, a valueindicative of a response of the actuator 204 to a command to operate theprocess control device 200 via the stem 202 is determined. In someexamples, the value indicative of the response of the actuatorcorresponds to a movement characteristic of the stem 202 in response toa force applied by the actuator 204 to the stem 202. The movementcharacteristic may be a sensitivity, a preciseness, or a responsivenessof movement of the stem 202. For example, the DVC 206 sends the commandto the actuator 204 (e.g., via a pneumatic signal) to operate theprocess control device 200. Based on the command, the actuator 204applies the force to the stem. If the friction increases or decreasesduring operation, the sensitivity, the preciseness, and/or theresponsiveness of the stem 202 may increase or decrease in response tothe force applied by the actuator 204 to the stem 202. As a result, theresponse of the actuator 204 to the command to operate the processcontrol device 200 may change or deteriorate. For example, if thefriction increases, the force to move the stem 202 via the actuator 204to a commanded position may increase.

In some examples, determining the value indicative of the response ofthe actuator 204 includes comparing the force corresponding to thefriction to an estimated force or torque. The estimated force or torquemay be a difference between an estimated maximum force or torque of theactuator 204 and an estimated force or torque of the actuator 204 toperform an action (e.g., move the flow control member, sealingly engageor disengage the flow control member and a seal, etc.). The estimatedmaximum force or torque of the actuator 204 is determined based oncharacteristics of the actuator 204, the stem 202 and/or the processcontrol system 100 such as, for example, a supply pressure available, anactuator effective area, a lever arm length, a maximum shear strength ofthe stem 202 and/or other characteristics. The estimated force or torqueof the actuator 204 to perform the action may be determined based onestimated process conditions (e.g., estimated valve inlet pressure,estimated valve outlet pressure) and/or characteristics of the actionsuch as, for example, shutoff classification desired, stem traveldistance, etc.

In some examples, the force corresponding to the friction is compared tothe estimated force using a ratio. For example, the ratio of theestimated force over the value corresponding to the friction may equal,for example, 15, which may indicate that the actuator 204 responds to acommand to move the stem 202 a distance corresponding to 0.25 percent oftotal possible stem travel by applying the first amount of force.However, if the ratio subsequently equals 5, the ratio may indicate thatthe response of the actuator 204 has deteriorated. For example, theratio equaling 5 may indicate that the friction of the actuator 204and/or the process control device 200 increased, and the actuator 204responds to the command to move the stem 202 the distance correspondingto 0.25 percent of the total possible stem travel by applying the secondamount of force that is greater than the first amount of force to thestem 202.

At block 406, it is determined whether the value indicative of theresponse of the actuator 204 indicates that the response of the actuator204 has deteriorated to or below a predetermined level. For example, thepredetermined level may be indicated by the ratio equaling 15. If thevalue indicates that the response of the actuator 204 has deterioratedto or below the predetermined level, then an alert message is sent(block 408). For example, the DVC 206 and/or the controller 104generates and sends the alert message to the workstation 108 if theratio equals 15 or less.

If the value indicates the response of the actuator 204 has notdeteriorated to or below the predetermined level, then a rate of changeof the value indicative of the response of the actuator 204 isdetermined (block 410). In some examples, determining the rate of changeof the value indicative of the response of the actuator 204 is based onthe value indicative of the response of the actuator 204 determined atblock 404 and one or more values indicative of the response of theactuator 204. For example, during operation, the process control device200 may undergo a plurality of operational cycles in which the actuator204 outputs a force to the stem 202. The DVC 206 and/or the controller104 may determine and/or log in a table or database a frequency of theforce outputs and a value indicative of the response of the actuator 204corresponding to the force output for each of the operational cycles.If, at block 406, the value indicates the response of the actuator 204determined at block 404 has not deteriorated to or below thepredetermined level then, at block 410, the frequency, the valuedetermined at block 404, and the values from the table or database maybe used to determine the rate of change of the value indicative of theresponse of the actuator 204. For example, corresponding to forceoutputs over about a five hour period of time, the values from the tableor database may be 16.5, 16.4, 16.3, and 16.15 and the value determinedat block 404 may be 16.0. As a result, the DVC 206 or the controller 104determines that the frequency of the force outputs is 1 per hour and therate of change of the value is, thus, about 0.1 per hour.

Based on the rate of change and the value indicative of the response ofthe actuator 204 determined at block 404, a time remaining until thevalue indicates the response of the actuator 204 has deteriorated to orbelow the predetermined level is determined (block 412). For example, ifthe value is 16, the rate of change of the value is 0.1 per hour and thepredetermined level is indicated by the value equaling 15, then the timeremaining until the value indicates the response of the actuator 204 hasdeteriorated to the predetermined level is 10 hours.

At block 414, whether the time is less than or equal to a predeterminedtime is determined. If the time is greater than the predetermined time,the example method 400 returns to block 402. If the time is equal to orless than the predetermined time, the alert message is sent (block 408).For example, the DVC 206 and/or the controller 104 generates and sendsthe alert message to the workstation 108 if the time is equal to or lessthan one day and/or any other suitable time.

An example method includes determining a force or torque correspondingto friction of a process control device and an actuator operativelycoupled to the process control device via a stem or shaft; anddetermining a value indicative of a response of the actuator to acommand to operate the process control device via the stem or shaftbased on the force or torque corresponding to the friction. In someexamples, determining the value indicative of the response of theactuator includes comparing the force or torque corresponding to thefriction to an estimated force or torque. In some examples, theestimated force or torque is based on a difference between an estimatedmaximum force or torque of the actuator and an estimated force or torqueof the actuator to perform an action. In some examples, the valueindicative of the response of the actuator corresponds to a movementcharacteristic of the stem or shaft in response to a force or torqueapplied to the stem or shaft by the actuator. In some examples, themovement characteristic is a sensitivity. In some examples, the movementcharacteristic is a preciseness. In some examples, the movementcharacteristic is a responsiveness. In some examples, the methodincludes sending an alert message when the value indicates the responseof the actuator has deteriorated to or below a predetermined level. Insome examples, determining the force or torque corresponding to thefriction includes determining a difference between a force or torqueapplied by the actuator to the stem or shaft at a first position and aforce or torque applied by the actuator to the stem or shaft at a secondposition during movement of the stem or shaft from the first position tothe second position.

An example method includes determining a value corresponding to frictionof an actuator and a process control device to be operated by theactuator; and determining a value indicative of an effect of thefriction on operation of the process control device via the actuatorbased on the value corresponding to the friction and a predeterminedvalue. In some examples, determining the value corresponding to thefriction includes determining a difference between a force or torqueapplied by the actuator to the stem or shaft at a first position and aforce or torque applied by the actuator to the stem or shaft at a secondposition during movement of the stem or shaft from the first position tothe second position. In some examples, the effect of the friction on theoperation of the process control device is an increase or decrease of asensitivity of movement of a stem or shaft of the process control devicein response to a force or torque applied by the actuator to the stem orshaft. In some examples, the effect of the friction on the operation ofthe process control device is a decrease of a preciseness of movement ofa stem or shaft of the process control device in response to a force ortorque applied by the actuator to the stem or shaft. In some examples,the effect of the friction on the operation of the process controldevice is an increase or decrease of a responsiveness of movement of astem or shaft of the process control device in response to a force ortorque applied by the actuator to the stem or shaft. In some examples,the predetermined value is a difference between a maximum force ortorque of the actuator and a force or torque of the actuator to performan action. In some examples, the method includes sending an alertmessage when the value indicative of the effect of the frictionindicates the effect of the friction has reached a predetermined level.In some examples, the method includes determining a rate of change ofthe value indicative of the effect of the friction. In some examples,the method includes determining the rate of change includes determininga frequency of force or torque outputs of the actuator.

An example tangible article of manufacture storing machine readableinstructions which, when executed, cause a machine to: determine a valuecorresponding to friction of an actuator and a process control device tobe operated by the actuator; and determine a value indicative of aneffect of the friction on operation of the process control device viathe actuator based on the value corresponding to the friction and apredetermined value. In some examples, the effect of the friction on theoperation of the process control device is a deterioration of a movementcharacteristic of a stem or shaft of the process control device inresponse to a force or torque applied by the actuator to the stem orshaft.

An example method includes determining a first force or torquecorresponding to friction of a process control device and an actuatoroperatively coupled to the process control device via a stem or shaft;in response to the first force or torque, determining a first command tooperate the process control device via the stem or shaft to obtain afirst response of the actuator; determining a second force or torquecorresponding to friction of the process control device and theactuator; and in response to the second force or torque, determining asecond command to operate the process control device via the stem orshaft to obtain a second response of the actuator.

In some examples, determining the first response of the actuator to thefirst command includes comparing the first force or torque correspondingto the friction to an estimated force or torque. In some examples, theestimated force or torque is based on a difference between an estimatedmaximum force or torque of the actuator and an estimated force or torqueof the actuator to perform an action. In some examples, the firstresponse of the actuator corresponds to a movement characteristic of thestem or shaft in response to a force or torque applied to the stem orshaft by the actuator. In some examples, the movement characteristic isa sensitivity. In some examples, the movement characteristic is apreciseness. In some examples, the movement characteristic is aresponsiveness. In some examples, the method includes sending an alertmessage when the first response or the second response of the actuatorhas deteriorated to or below a predetermined level.

In some examples, determining the first force or torque corresponding tothe friction includes determining a difference between a force or torqueapplied by the actuator to the stem or shaft at a first position and aforce or torque applied by the actuator to the stem or shaft at a secondposition during movement of the stem or shaft from the first position tothe second position. In some examples, the second command is modifiedfrom the first command to compensate for differences between the firstand second forces or torques.

An example tangible article of manufacture storing machine readableinstructions which, when executed, cause a machine to: determine a forceor torque corresponding to friction of a process control device and anactuator operatively coupled to the process control device via a stem orshaft; in response to the force or torque, determine a first command tooperate the process control device via the stem or shaft to obtain aresponse of the actuator; determine a second force or torquecorresponding to friction of the process control device and theactuator; and in response to the second force or torque, determine asecond command to operate the process control device via the stem orshaft to obtain a second response of the actuator.

An example method includes determining a force or torque at a pluralityof times during a continuous process control operation, the forces ortorques corresponding to friction of a process control device and anactuator operatively coupled to the process control device via a stem orshaft; and in response to the forces or torques at the respective times,dynamically updating a command to operate the process control device viathe stem or shaft to obtain a response of the actuator during thecontinuous process control operation.

In some examples, determining the response of the actuator to thecommand includes comparing the force or torque corresponding to thefriction to an estimated force or torque. In some examples, theestimated force or torque is based on a difference between an estimatedmaximum force or torque of the actuator and an estimated force or torqueof the actuator to perform an action. In some examples, the response ofthe actuator corresponds to a movement characteristic of the stem orshaft in response to a force or torque applied to the stem or shaft bythe actuator. In some examples, the movement characteristic is one ormore of a sensitivity, a preciseness, or a responsiveness. In someexamples, the method includes sending an alert message when the responseof the actuator has deteriorated to or below a predetermined level. Insome examples, determining the force or torque corresponding to thefriction includes determining a difference between a force or torqueapplied by the actuator to the stem or shaft at a first position and aforce or torque applied by the actuator to the stem or shaft at a secondposition during movement of the stem or shaft from the first position tothe second position.

Although certain example methods and apparatus have been describedherein, the scope of coverage of this patent is not limited thereto. Onthe contrary, this patent covers all methods, apparatus and articles ofmanufacture fairly falling within the scope of the appended claimseither literally or under the doctrine of equivalents.

The Abstract at the end of this disclosure is provided to comply with 37C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature ofthe technical disclosure. It is submitted with the understanding that itwill not be used to interpret or limit the scope or meaning of theclaims.

What is claimed is:
 1. An apparatus, comprising: a housing; a processordisposed in the housing, the processor to: determine a first force ortorque corresponding to friction of a process control device and anactuator operatively coupled to the process control device via a stem orshaft; in response to the first force or torque, determine a firstcommand to operate the process control device via the stem or shaft toobtain a first response of the actuator; determine a second force ortorque corresponding to friction of the process control device and theactuator; and in response to the second force or torque, determine asecond command to operate the process control device via the stem orshaft to obtain a second response of the actuator.
 2. The apparatus ofclaim 1, wherein the processor is to determine the first response of theactuator to the first command by comparing the first force or torquecorresponding to the friction to an estimated force or torque.
 3. Theapparatus of claim 2, wherein the processor is to determine theestimated force or torque based on a difference between an estimatedmaximum force or torque of the actuator and an estimated force or torqueof the actuator to perform an action.
 4. The apparatus of claim 1,wherein the processor is to determine the first response of the actuatorbased on a movement characteristic of the stem or shaft in response to aforce or torque applied to the stem or shaft by the actuator.
 5. Theapparatus of claim 4, wherein the movement characteristic includes oneor more of a sensitivity, a preciseness, or a responsiveness.
 6. Theapparatus of claim 1, wherein the processor is to send an alert messagewhen the first response or the second response of the actuator hasdeteriorated to or below a threshold level.
 7. The apparatus of claim 1,wherein the processor is to determine the first force or torquecorresponding to the friction by determining a difference between aforce or torque applied by the actuator to the stem or shaft at a firstposition and a force or torque applied by the actuator to the stem orshaft at a second position during movement of the stem or shaft from thefirst position to the second position.
 8. The apparatus of claim 1,wherein the processor is to modify the second command from the firstcommand to compensate for differences between the first and secondforces or torques.
 9. An apparatus, comprising: a housing; a processordisposed in the housing, the processor to: determine a force or torqueat a plurality of times during a continuous process control operation,the forces or torques corresponding to friction of a process controldevice and an actuator operatively coupled to the process control devicevia a stem or shaft; and in response to the forces or torques at therespective times, dynamically update a command to operate the processcontrol device via the stem or shaft to obtain a response of theactuator during the continuous process control operation.
 10. Theapparatus of claim 9, wherein the processor is to determine the responseof the actuator to the command by comparing the force or torquecorresponding to the friction to an estimated force or torque.
 11. Theapparatus of claim 10, wherein the processor is to determine theestimated force or torque based on a difference between an estimatedmaximum force or torque of the actuator and an estimated force or torqueof the actuator to perform an action.
 12. The apparatus of claim 11,wherein the processor is to determine the response of the actuator basedon a movement characteristic of the stem or shaft in response to a forceor torque applied to the stem or shaft by the actuator.
 13. Theapparatus of claim 12, wherein the movement characteristic is one ormore of a sensitivity, a preciseness, or a responsiveness.
 14. Theapparatus of claim 9, wherein the processor is to generate an alertmessage when the response of the actuator has deteriorated to or below athreshold level.
 15. The apparatus of claim 9, wherein the processor isto determine the force or torque corresponding to the friction bydetermining a difference between a force or torque applied by theactuator to the stem or shaft at a first position and a force or torqueapplied by the actuator to the stem or shaft at a second position duringmovement of the stem or shaft from the first position to the secondposition.